Quantum key distribution (QKD) endeavors to generate and distribute cryptographic keys using quantum physics in order to provide a higher level of security in communications than can be accomplished using classical means. There are significant challenges that must be overcome to accomplish this goal in practice.
One challenge common to QKD and other communication methods are the speed of the sender's laser modulation, speed of receiver's detectors, and reasonable assurance of having a single photon.
An eavesdropper may be near the sender, and may be able to electronically detect the modulated power drawn by the sender's lasers, or be able to read the pseudo-random number generator the sender uses to choose frames. The physical security of the sender and receiver is often overlooked. Other challenges may include power, cost, weight and robustness of operation.